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#27804 - 08/01/03 03:12 PM Light Contactor humming.
Sandro Offline

Registered: 12/30/01
Posts: 449
Loc: Stoney Creek, ON, Canada
Got a service call today, contactor #5 (there are a total of 6) humming/buzzing loudly. Traced it to coil. When I tapped it, I got different levels of 'hum', however, at its highest level it was quite annoying.

Anyways, what caused it to hum? Is this common to contactors?

Here are particulars if anybody cares... Square D product. 120V coil, and 3 pole 40amp rated, but only 15A @ 120V max actual load per pole.


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#27805 - 08/01/03 05:18 PM Re: Light Contactor humming.
Wirenuttt Offline

Registered: 11/10/02
Posts: 267
Loc: Massachusetts
I think the first thing I would look at is check for proper neutral connection to the coil. Check with amprobe under load. Make sure coil is sitting properly. Tighten all connections. Compare wire gauge to fuse, breaker and load.

#27806 - 08/01/03 05:29 PM Re: Light Contactor humming.
NJwirenut Offline

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
Contactors will often hum if dirt or rust gets between the pole faces of the magnetic core. A cleaning with a clean rag or fine sandpaper will often cure this.

#27807 - 08/01/03 06:41 PM Re: Light Contactor humming.
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Solenoid Coils for Contactors / Relays typically begin making obnoxious 120 Hz humming noise due to:

    [*]Contaminates between core laminations,

    [*]Loose core laminations,

    [*]Mis-Alignment of Solenoid,

    [*]Contacts not seating completely (aka "chatter").

As mentioned, if the laminated core of the Contactor's Solenoid Coil has dirt build up or rust between laminations, this will cause humming (120 Hz humming to be precise).
This could occur on the stationary part, the moving / sliding part, or both.

If the laminations become loosened, this also will cause noisy operation.

This also could occur on the stationary part, the moving / sliding part, or both.

If the contacting "face" points of the core sections do not line up evenly, have excessive build-up on them, are slightly twisted off axis, or if one (or both) side(s) doesn't "pull-up" completely- resulting in a slight gap between stationary and moving cores when the Contactor is locked in, this will cause noise and heat.

If the sliding portion of the Contactor does not completely lock-in, this will cause excessive noise and heat.

If you have the time (and patients ) to clean it, try that.
Need to remove the cores from around the coil and from the sliding assembly.
Clean cores with appropriate solution, let dry completely (maybe even dry in an oven at the lowest setting!), then re-assemble.

On a mis-aligned contactor, try loosening up the mounting screws during operation to see if this causes a drammatic reduction in noise. If so, re-mount contactor as needed.
Be sure to include isolation materials between contactor and enclosure!

For extremely mis-aligned contactors (out of alignment due to someone / something hitting it really hard, or installing it across an uneven surface), or contactors with a seating problem, there may be only one thing to do - replace the entire contactor!

Also, for severly rusted / loosened laminations, might need to replace core(s), contact assembly or complete contactor!

Good luck.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#27808 - 08/01/03 08:06 PM Re: Light Contactor humming.
Trumpy Offline


Registered: 07/05/02
Posts: 8532
Loc: SI,New Zealand
I'd agree with NJwirenut on that one,
it's more a mechanical thing than an Electrical problem.
I'd say replace it, with a new one and save yourself the hassle of repeat call-outs.
Let's face it, these days if you're not young, you're old - Red Green grin

#27809 - 08/01/03 11:12 PM Re: Light Contactor humming.
frank Offline

Registered: 05/29/01
Posts: 361
Loc: windsor ontario canada
I agree with Scott35 and NJwirenut too.i think this condition causes the coil to burn up over time as well.I'd change it for fear of a call back.

[This message has been edited by frank (edited 08-02-2003).]

#27810 - 08/02/03 04:40 AM Re: Light Contactor humming.
Electricmanscott Offline

Registered: 01/12/02
Posts: 1478
Loc: Holden, MA USA
"Why does the contactor hum"? It doesn't know the words! Thank you I'll be here all week.... But seriously just replace it.

#27811 - 08/02/03 02:21 PM Re: Light Contactor humming.
n1ist Offline

Registered: 02/13/02
Posts: 176
Loc: Malden MA
Also make sure the contactor is securely mounted in the box.

#27812 - 08/02/03 03:25 PM Re: Light Contactor humming.
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Lapping the {often E/I-shaped} pole faces with something like 320-grit abrasive paper on a smooth surface is a usual fix. It is typically not a good idea to use any type of lubricant on the mechanism, for airborne contaminants are readily attracted.

Off-cycle condensation in contactor enclosures will assure repeat business.

#27813 - 08/02/03 06:30 PM Re: Light Contactor humming.
ThinkGood Offline

Registered: 08/07/02
Posts: 1084
Loc: Milwaukee, WI

Why 120Hz?

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